Synthesis of Azocane- and Oxocane-Annulated Furans by a [2+2] Photocycloaddition-Ring-Opening Cascade
Abstract
The title compounds were synthesized from readily available quinolone and coumarin derivatives by a cascade reaction (12 examples, 90-98% yield). The cascade comprised a [2+2] photocycloaddition which occurred upon sensitized irradiation at lambda = 420 nm (or direct UV irradiation at lambda = 366 nm) and a subsequent acid-catalyzed ringopening reaction. A variety of substituents are compatible with the conditions and a 3-alkyl group in the coumarin (or quinolone) is crucial to achieve a high chemoselectivity. Key to the success of the ring opening is the formation of a 4,5,5a-trihydrocyclobuta-2H-furan containing a strained bridgehead double bond which stems from the allenyl group tethered to the 4-position of the starting materials.
